1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy having a BCC structure (body-centered cubic structure) as a crystal structure and, more particularly, to a hydrogen-absorbing alloy for a nickel-hydride cell having an excellent discharge capacity and excellent durability (cycle characteristics).
2. Description of the Prior Art
A hydrogen-absorbing alloy can absorb and store a volume of hydrogen gas more than about 1,000 times the volume of the alloy itself as means for storing and transporting hydrogen, and its density is equal to, or greater than, that of liquid or solid hydrogen. It has long been known that metals and alloys having a body-centered structure (hereinafter called the xe2x80x9cBCCxe2x80x9d), such as V, Nb, Ta, TiVMn system and TiVCr system alloys absorb and store greater amounts of hydrogen than AB5 type alloys such as LaNi5 and AB2 type alloys such as TiMn2 that have been already put into practical application. This is because the number of hydrogen absorbing sites in the crystal lattice of the BCC structure is large, and the hydrogen-absorbing capacity according to calculation is as great as H/M=2.0 (about 4.0 wt % in alloys of Ti or V having an atomic weight of about 50).
Though hydrogen-absorbing alloys have been used for cell electrodes in this field, the number of alloys having a body-centered cubic structure (BCC) is small, and Laves phase alloys of the AB2 type such as misch metal have been mainly disclosed.
Japanese Unexamined Patent Publication (Kokai) No. 6-228699 discloses a hydrogen-absorbing alloy for an electrode of an alkali secondary cell which is expressed by the formula TixVyNiz and the composition range of which falls within the range encompassed by Ti5V90Ni5, Ti5V75Ni20, Ti30V50Ni20 and Ti30V65Ni5. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-268514 discloses a hydrogen-absorbing alloy, and a hydrogen-absorbing alloy electrode, wherein a phase comprising the AB2 type Laves alloy phase, as the principal phase, exists while it forms a three-dimensional stitch skeletal structure in the base phase comprising a Ti-V type solid solution alloy, and Japanese Unexamined Patent Publication (Kokai) No. 9-49046 discloses a hydrogen-absorbing alloy, and an electrode, expressed by the general formula TixVyMzNi1-x-y-z (where M is at least one element selected from the group consisting of Cr, Mo and W, and 0.2xe2x89xa6xxe2x89xa60.4, 0.3xe2x89xa6y less than 0.7, 0.1xe2x89xa6z xe2x89xa60.3 and 0.6xe2x89xa6x+y+zxe2x89xa60.95), and having a body-centered cubic structure. Japanese Unexamined Patent Publication (Kokai) No. 9-53135 describes a hydrogen-absorbing alloy, and an electrode, expressed by the general formula TixVyNi1-x-y-z (where M is at least one kind of element selected from the group consisting of Co, Fe, Cu and Ag, and 0.2xe2x89xa6xxe2x89xa60.4, 0.3xe2x89xa6y less than 0.7, 0.1xe2x89xa6zxe2x89xa60.3 and 0.6xe2x89xa6x+y+zxe2x89xa60.95) and having a body-centered cubic structure. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 9-53136 describes a hydrogen-absorbing alloy, and an electrode, expressed by the general formula TixVyMzNi1-x-y-z (where M is at least one kind of the element selected from the group consisting of Al, Mn and Zn, 0.2xe2x89xa6xxe2x89xa60.4, 0.3xe2x89xa6y less than 0.7, 0.1xe2x89xa6zxe2x89xa60.3 and 0.6xe2x89xa6x+y+zxe2x89xa60.95), and having a body-centered cubic structure.
Further, Japanese Unexamined Patent Publication (Kokai) No.9-53137 describes a hydrogen-absorbing alloy, and an electrode, expressed by the general formula TixVyMzNi1-x-y-z (where M is at least one kind of element selected from the group consisting of Zr and Hf, 0.2xe2x89xa6xxe2x89xa60.4, 0.3xe2x89xa6y less than 0.7, 0.1xe2x89xa6zxe2x89xa60.3 and 0.6xe2x89xa6x+y+zxe2x89xa60.95), and having a body-centered cubic structure.
However, all these BCC alloys contain large amounts of V and their durabilities (cycle characteristics) are not sufficient.
In order to convert the Tixe2x80x94Vxe2x80x94Cr type alloy, as one of the conventional BCC type hydrogen-absorbing alloys, to a quaternary or quinary alloy having a periodical structure by substituting V in the Tixe2x80x94Vxe2x80x94Cr alloy with other element and controlling the lattice constant, the present invention aims at providing a hydrogen-absorbing alloy, and an electrode, that can be used for cells having excellent cycle characteristics.
Another object of the present invention is to make it possible to produce an alloy, which is advantageous from the aspect of the production cost and has excellent hydrogen absorption and desorption characteristics, by heat-treatment, and to provide a hydrogen-absorbing alloy, and an electrode, that can be applied on the industrial scale to Ni-MH (Metallic hydride) cells.
Another object of the present invention is to provide an alloy for cells, which has a periodical structure by a spinodal decomposition and can be produced at a low cost on the industrial scale, by using the novel BCC alloy and heat-treatment described above through an optimum production process.
The gist of the present invention will be described as follows.
(1) A hydrogen-absorbing alloy comprises a composition expressed by the general formula:
Ti (100xe2x88x92axe2x88x92bxe2x88x92cxe2x88x92d)CraVbNicXd,
where X is at least one member selected from the group consisting of Y (yttrium), lanthanoids, Pd and Pt, each of a, b, c and d is represented, in terms of atomic %, by the relations 8xe2x89xa6axe2x89xa650, 30 less than bxe2x89xa660, 5xe2x89xa6cxe2x89xa615, 2xe2x89xa6dxe2x89xa610 and 40xe2x89xa6a+b+c+dxe2x89xa690;
and a crystal structure of a principal phase which is a body-centered cubic structure.
(2) A hydrogen-absorbing alloy comprises a composition expressed by the general formula:
Ti (100xe2x88x92axe2x88x92bxe2x88x92cxe2x88x92d) CraVbNicXd,
where X is at least one member selected from the group consisting of Y (yttrium), lanthanoids, Pd and Pt and each of a, b, c and d is represented, in terms of atomic %, by the relations 8xe2x89xa6axe2x89xa650, 0 less than bxe2x89xa630, 5xe2x89xa6cxe2x89xa615, 2xe2x89xa6dxe2x89xa610 and 40xe2x89xa6a+b+c+dxe2x89xa690;
and a crystal structure of a principal phase which is converted to a body-centered cubic structure by heat-treatment.
(3) A hydrogen-absorbing alloy comprises a composition expressed by the general formula:
Ti (100xe2x88x92axe2x88x92bxe2x88x92cxe2x88x92d) CraMbNicXd,
where M is at least one of Mo and W, X is at least one member selected from the group consisting of Y (yttrium), lanthanoids, Pd and Pt, and each of a, b, c and d is expressed, in terms of atomic %, by the relations 8xe2x89xa6axe2x89xa650, 30 less than bxe2x89xa660, 5xe2x89xa6cxe2x89xa615, 2xe2x89xa6dxe2x89xa610 and 40xe2x89xa6a+b+c+dxe2x89xa690;
and a crystal structure of a principal phase which is converted to a body-centered cubic structure by heat-treatment.
(4) A hydrogen-absorbing alloy having the composition according to any of above items 1 through 3, wherein the principal phase exists within the range where a body-centered cubic structure appears and a spinodal decomposition occurs, exclusive of a C14 single-phase region, where C14 is a typical structure of a Laves phase and MgZn2 type crystal structure; and said principal phase has a regular periodical structure and its apparent lattice constant is from 0.2950 nm to 0.3150 nm.
(5) A hydrogen-absorbing alloy according to item (2) or (3), wherein heat-treatment comprises solution treatment conducted for 1 min to 100 hr at a temperature range of from 700 to 1500xc2x0 C., and one or both treatments selected from quenching and aging of from 350 to 1200xc2x0 C. after solution treatment.
(6) A cell electrode comprising said hydrogen-absorbing alloy according to any one of items (1) through (4).
(7) A cell electrode according to item (6), wherein said cell electrode has excellent cell characteristics in the maximum discharge capacity and the capacity retaining ratio after 100 charge/discharge cycles.
(8) A cell electrode according to item (7), wherein the maximum discharge capacity is 375 to 465 mAh/g and the capacity retaining ratio after 100 charge/discharge cycles is 80 to 95%.